The US Department of the Interior says climate change will cut water flow in the west of the country causing water shortage for millions in the region.

According to a report released by the department on Monday, global warming will reduce water levels in several of the American West's largest river basins by up to 20% this century, the state-funded BBC reported.

The endangered rivers, which include The Colorado, the Rio Grande and the San Joaquin, provide water to eight US states, from Texas to California.

The three rivers will face an 8% to 20% decrease in average annual stream flow, says the report, which has been prepared in response to the Secure Water Act of 2009.

"Impacts to water are on the leading edge of global climate change," said Mike Connor, commissioner of the Bureau of Reclamation, a US agency that helps provide water to more than 31 million people in 17 western states.

Interior Secretary Ken Salazar said reduced amounts of water or in his words the region's "lifeblood," would have a serious effect on the west and south west, which are among the fastest-growing regions of the United States.

"These changes will directly affect the west's water supplies, which are already stretched in meeting demands for drinking, irrigating crops, generating electricity and filling our lakes and aquifers for activities like fishing, boating and to power our economy," Salazar told Reuters.

The report also says that the northwestern and north-central portions of the western US will witness rainfall increase, while the southwestern and south-central areas will experience a decrease in precipitation.

Climate changes will affect water supplies to a wide range of users, from farms and cities to hydropower plants, fish, wildlife and recreation, the report said.

"As climate change adds to the challenges we face in managing our water supply, meaningful engagement between the river basin states and the Department of the Interior will continue to be essential," said Anne Castle, Assistant Secretary of the Interior for Water and Science.

Salazar believes the report will help officials understand the long-term impacts of climate change on water supplies and develop strategies for sustainable water resource management.